The long term goal of this proposal is to understand the role of oxygen and tissue hypoxia in the pathogenesis and treatment of diabetric retinopathy. This study will investigate the presence of retinal hypoxia in human diabetic retinopathy; and the way panretinal photocoagulation affects the retinal oxygen tension and at the same time affect neovascularizaton. We will investigate: 1. Is retinal hypoxia present in human proliferative diabetic retinopathy? 2. Does panretinal photocoagulation relieve retinal hypoxia in human diabetic retinopathy? These goals will be achieved through the use of an intraocular fiber optic oxygen sensor during intraocular surgery, to measure the preretinal oxygen tension in (1) human patients with proliferative diabetic retinopathy, (2) non-diabetic controls, and (3) in patients with proliferative diabetic retinopathy where measurements can be made on adjacent laser-treated and untreated retinal areas in the same eye. It has been suggested that retinal hypoxia leads to the formation of a vasoproliferative substance that causes retinal neovascularization. This hypothesis has held center stage in diabetic retinopathy research for 30 years, but the presence of retinal hypoxia has not been verified directly. This project will provide the first data on human retinal oxygen tension. Animal studies have shown that panretinal photocoagulation improves inner retina oxygen supply. This study will investigate whether this is true in human diabetics. If panretinal photocoagulation reverses hypoxia in diabetic retinas this could be its therapeutic mechanisms.